Comparative Electrochemical and Mechanical Investigation of Alkali Ion Insertion into Berlin Green

Stable electrode materials with reversible ion insertion/extraction are central to developing sodium- and potassium-ion batteries as lithium-ion alternatives. Here, we investigated the impact of different monovalent insertion ions on the performance and reliability of the same electrode. We chose Berlin Green (BG, Fe$^{III}$[Fe$^{III}$(CN)$_6$]), a Prussian Blue analog (PBA), due to its rigid structure, ability to accommodate different ion sizes, and two different redox-active Fe atoms. We conducted electrochemical tests, intermittent scanning electron microscopy (SEM), and operando substrate curvature experiments on BG as we changed the counter electrode and electrolyte. With this novel experimental strategy, we report for the first time sequential insertion/extraction of all three ions in/from the same single BG electrode, eliminating electrode variations and ensuring direct comparability of ion insertion, degradation, and stress. While Li$^+$ and Na$^+$ follow comparable pathways, K$^+$ leads to distinct electrochemical and mechanical responses, underscoring ion-specific interactions within the host framework. We demonstrate that BG enables highly reversible insertion/extraction with a high tolerance to ionic size variation and great surface stability, despite volume changes and phase transitions. These results position BG as a model system for understanding alkali ion insertion and as a versatile and sustainable electrode material for future energy storage systems.